Accurate estimates of daily manure production per animal are essential for designing livestock facilities and developing environmental policies. This study assessed manure production in bedded pack barns, a common type of beef cattle housing in Korea, where excreted manure is mixed with bedding and composted before removal. Because composting alters manure mass through moisture evaporation and organic matter decomposition, manure production was evaluated by season (summer, autumn, winter) and cattle growth stage (growing calf, steer, cow). A reliable baseline was established by directly measuring raw manure excretion in a pen for 12 days under controlled conditions (average temperature of −3.9°C and a wind speed of 0.1 m/s), under which moisture evaporation and organic matter decomposition were negligible. Average manure production varied by growth stage: 11.8 ± 2.9 kg/head/day for growing calf, 11.2 ± 2.7 kg/head/day for steer, and 15.8 ± 4.7 kg/head/day for cow. Manure production showed a significant correlation with feed intake (R2 = 0.69), which partially explained the variation across growth stages; however, composting processes influenced by seasonal factors also played important roles. Compared to the baseline, moisture mass decreased by 50%–55% in summer, 12%–22% in autumn, and 6%–21% in winter. Volatile solids decomposition ranged seasonally from 37% to 48% in summer to 9% to 14% in winter, with autumn values between 22% and 28%. Consequently, total manure production declined by approximately 51% in summer and 18% in winter relative to the baseline. These findings underscore the importance of considering seasonal effects, growth stages, and composting practices when estimating manure production in bedded pack barns.